For a very long time there have been efforts to produce environmentally stable coatings and devices having very low reflectivity for a variety of industrial and scientific applications. They are important in imaging systems, calibration targets, instrumentation, light guides, baffles, stray light suppression and many other uses.
To be commercially useful, these coatings must have a low reflectance, but as important, they should exhibit the following: be spectrally flat, low outgassing, low particulate fallout, thermal shock resistance, resistance to moisture and high resistance to shock and vibration. These can be key requirements, as the coatings are often local to high sensitivity electronic detectors such as CCD or micro bolometers. Any contamination from such coatings will inevitably collect or condense on the detectors rendering them faulty or lowering their performance beyond an acceptable threshold.
Until recently, the best spray applied coatings have achieved a reflectivity of around 2.5% in the visible spectrum (380 nm-760 nm wavelength), although some experimental studies have achieved better results by using CVD grown, aligned carbon nanostructures, for instance around 0.045 to 0.5% total hemispherical reflectance (THR) when deposited on small lab scale substrates. One example of an aligned absorber is: US patent application: 2009/0126783 by Shawn-Yu Lin et al of Rensselaer Polytechnic Institute, entitled: “Use of vertical aligned carbon nanotube as a super dark absorber for pv, tpv, radar and infrared absorber application”. This document discusses a visible spectrum, highly absorbing aligned carbon nanotube film. Whilst interesting, these aligned array absorbers are grown at high temperatures >750° C. using complex and costly chemical vapour deposition (CVD) reactors and require even more complex catalyst steps created in Physical Vapour Deposition (PVD) reactors. This limits their use to specialist substrates with simple planar geometries that that are capable of fitting into existing reactors, thereby limiting their commercial applications to small, simple substrates that can tolerate the high temperatures (>750° C.) used during growth of the carbon nanotubes. Also, due to the CVD method used to grow these films, they tend to be very hydrophilic as, the present inventors have discovered, growth defects in the tube wall terminate to form highly polar hydroxyl, carbonyl and carboxyl functional groups on exposure to air. This hydrophilicity rapidly causes the film to lose its optical properties on exposure to atmospheric humidity or free water as it acts like a sponge.
A study by John H Lehman et al, “Single-Wall Carbon Nanotube Coating on a Pyroelectric Detector”, Applied Optics, 1 Feb. 2005 vol. 44, No 4, has suggested that a low reflectivity coating formed from a solution of carbon nanotubes and a suitable solvent could exhibit high levels of absorbance when applied to pyroelectric detectors. These films created from a solvent/carbon nanostructure solution have shown that they are only able to achieve a total hemispherical reflectance (THR) of around 2%, which is only on a par with the best existing commercial black paints. This is due to the high density of the applied film resulting in multiple carbon nanotube sidewalls that act as an effective reflector. This allows incoming photons to be reflected without being absorbed. The sprayed coating is also hydrophilic and so suffers from the same atmospheric contamination issues as aligned array films. The coating also suffers from poor substrate adhesion.
In fields unrelated to optical absorbers, researchers have created solutions of solvent dispersed, functionalised carbon nanotubes for electronic ink applications. In this type of application, it is desired that the ink be stable, printable and have low electrical resistance after printing. An example patent is US-2013/0273257, “Carbon Nanotube Ink Composition and a Coating Method Thereof and a Forming Method of a Thin Film Containing Carbon Nanotubes”. This document discloses the creation of functionalised carbon nanotubes in a solvent solution capable of being inkjet-printed. These types of carbon nanostructure inks do not make good optical absorbers as the functionalisations and surfactants used all contribute spectral features related to the chemical bonds in the surfactant that contribute to a large increase THR across critical parts of the electromagnetic spectrum.
It is also known that research groups have created super hydrophobic carbon nanotube films by depositing fluorocarbon or organosilanes on top of previously grown carbon nanotube films and powders. An example is described by Kenneth K S Lau, in “Nano Letters—Super Hydrophobic Carbon Nanotube Forests”. This type of coating will prevent the aligned array or carbon nanotube powder from taking up moisture, but the hydrophobic coating thicknesses required to do so will reduce the film's absorbance due to the large difference in refractive index of the continuous hydrophobic coating, that blocks of the open space between the tubes, and the fact that the carbon nanotube/filament has a reduced ability to absorb photons when fully covered by a polymeric layer.
Prior art which discloses various methods of coating carbon nanostructures on substrates includes the following: US2016194205A; US2016053155A; US2014272199A; US2013316482A; US2013194668A; US2015298164A; US2013273257A; US2011315981A; US2011217544A; US2011111177A; US2010230344A; US2009026424A; US2008083950A; US2005013935A; and US2001004471A. However, none of these documents disclose any steps which result in ultra-low reflectivity coatings.
Other prior art documents include WO2003/086961 A2 (DU PONT); WO2007/075437 A2 (INTEMATIX); US2008/0171193 A1 (YI); WO2009/058763 A1 (UNIDYM); US2009/0050601 A1 (PARK); and CN104558659 A (U. BEIJING).